Detection circuit for an active discharge circuit of an X-capacitor, related active discharge circuit, integrated circuit and method

Abstract

A method and apparatus for an active discharge of an X-capacitor are provided. A sensor signal, indicative of a voltage at the capacitor, is compared with a lower and upper threshold values. A first value of a smaller one of the lower and upper threshold values is increased to a first new value that is greater than a second value of a larger one of the lower and upper threshold values in response to a first control signal indicating the sensor signal is greater than the upper and lower threshold values. A third value of the greater one of the lower and upper threshold values is decreased to a second new value that is less than the value of the larger one of the lower and upper threshold values in response to a second control signal indicating the sensor signal is less than the upper and lower threshold values.

Claims

1. A device, comprising: a discharge circuit; a capacitor coupled to the discharge circuit; and a detection circuit coupled to the discharge circuit and including: a sensor circuit coupled to the capacitor; a comparator configured to: compare a sensor signal from the sensor circuit with a lower threshold value and determine whether the sensor signal is greater than the lower threshold value; and compare the sensor signal with an upper threshold value and determine whether the sensor signal is lower than the upper threshold value; an elaboration circuit configured to generate a first control signal indicating whether the sensor signal is increasing and a second control signal indicating whether the sensor signal is decreasing; and a dynamic threshold generator circuit configured to: vary the lower threshold value and the upper threshold value of the comparator as a first function of the first control signal to increase a first value of a smaller one of the lower threshold value and the upper threshold value to a first new value that is greater than a value of a larger one of the lower threshold value and the upper threshold value in response to the first control signal; and vary the lower threshold value and the upper threshold value of the comparator as a second function of the second control signal to decrease a second value of the larger one of the lower threshold value and upper threshold value to a second new value that is less than the smaller one of the lower threshold value and the upper threshold value in response to the second control signal.

2. The device of claim 1 wherein the comparator further includes: a first comparator configured to generate a first comparison signal indicating whether the sensor signal is greater than the lower threshold value; and a second comparator configured to generate a second comparison signal indicating whether the sensor signal is greater than the upper threshold value.

3. The device of claim 2 wherein the elaboration circuit is further configured to: generate, based on the first comparison signal and the second comparison signal from the comparator, the first control signal indicating whether the sensor signal is greater than both of the upper threshold value and lower threshold value; and generate, based on the first comparison signal and the second comparison signal from the comparator, the second control signal indicating whether the sensor signal is smaller than both of the upper threshold value and the lower threshold value.

4. The device of claim 3, further comprising a timer circuit configured to: set a discharge enable signal to a first logic level in response to the timer circuit being reset via a reset signal; determine a time elapsed since the timer circuit has been reset via the reset signal; and in response to the time elapsed exceeding a timeout value, set the discharge enable signal to a second logic level to discharge the capacitor with the discharge circuit.

5. The device of claim 4 wherein the timer circuit is configured to reset the timer circuit with the reset signal in response to the first comparison signal and the second comparison signal indicating that the sensor signal is greater than both of the first threshold value and second threshold value.

6. A method, comprising: detecting an AC oscillation at a capacitor by: comparing a sensor signal, indicative of a voltage at the capacitor, with a lower threshold value and determining whether the sensor signal is greater than the lower threshold value: comparing the sensor signal with an upper threshold value and determining whether the sensor signal is lower than the upper threshold value; increasing a first value of a smaller one of the lower threshold value and the upper threshold value to a first new value that is greater than a second value of a larger one of the lower threshold value and the upper threshold value in response to a first control signal indicating a sensor signal is greater than the values of the upper threshold value and the lower threshold value; and decreasing a third value of the greater one of the lower threshold value and the upper threshold value to a second new value that is less than the value of the larger one of the lower threshold value and the upper threshold value in response to a second control signal indicating the sensor signal is less than the values of the upper threshold value and the lower threshold value.

7. The method of claim 6 wherein detecting that the AC oscillation includes: detecting at least one of a leading edge or a falling edge of a comparison signal that is indicative of whether the sensor signal is between the lower threshold and the upper threshold values; incrementing a count value indicating a time between the at least one of the leading edge or the falling edge of the comparison signal; and determining whether the count value has reached a time threshold value.

8. The method of claim 7, further comprising discharging the capacitor in response to the count value reaching the time threshold value.

9. A device, comprising: a discharge capacitor; a voltage sensor coupled to the discharge capacitor and configured to output a signal representative of a voltage of the discharge capacitor; a first comparator coupled to the voltage sensor and configured to compare the signal to a first threshold; a second comparator coupled to the voltage sensor and configured to compare the signal to a second threshold; an elaboration circuit coupled to the first and second comparators, the elaboration circuit including: an AND gate coupled to the first and second comparators; and a NOR gate coupled to the first and second comparators, wherein the AND gate and the NOR gate together indicate whether the signal is greater than, less than or between the first and second thresholds; and a dynamic threshold generator circuit coupled to the first and second comparators and to the elaboration circuit.

10. The device of claim 9 wherein the dynamic threshold generator circuit is coupled to an output of the AND gate and an output of the NOR gate.

11. The device of claim 9 further comprising a timer circuit coupled to the elaboration circuit.

12. The device of claim 11 wherein the timer circuit is coupled to an output of the AND gate.

13. The device of claim 9, further comprising a rectifier coupled between the discharge capacitor and the voltage sensor, the rectifier including: a first diode coupled to the discharge capacitor; a second diode coupled to the discharge capacitor; a first resistor coupled to the first and second diode; and a second resistor.

14. The device of claim 13 wherein a node between the first and second resistor is coupled to an input of each of the first and the second comparators.

15. The device of claim 9 further comprising a discharge circuit coupled to the capacitor, the discharge circuit including: a first diode coupled to the capacitor; a second diode coupled to the capacitor; and a switch coupled to the first and second diode.

16. The device of claim 15 further comprising a timer circuit coupled to the elaboration circuit, the timer circuit is coupled to an output of the AND gate, and the switch is coupled to an output of the timer circuit.

17. The device of claim 9 wherein the dynamic threshold generation circuit further includes: a processing unit coupled to the AND gate and to the NOR gate; a first digital to analog converter coupled to the processing unit; and a second digital to analog converter coupled to the processing unit.

18. The device of claim 17 wherein an output of the first digital to analog converter is coupled to the second comparator and an output of the second digital to analog converter is coupled to the first comparator.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Embodiments of the present disclosure will now be described with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

(2) FIGS. 1a and 1b are functional diagrams of devices including conventional discharge circuits;

(3) FIG. 2 shows a device comprising an X-capacitor in accordance with one embodiment of the present disclosure;

(4) FIG. 3 shows an embodiment of an active discharge circuit for the device of FIG. 2;

(5) FIG. 4 shows an embodiment of a detection circuit for the active discharge circuit of FIG. 3;

(6) FIGS. 5a and 5b show typical waveforms occurring in the detection circuit of FIG. 4;

(7) FIG. 6 shows a first embodiment of a processing unit of the detection circuit shown in FIG. 4;

(8) FIGS. 7, 8 and 9 show waveforms which may occur in the processing unit of FIG. 6;

(9) FIG. 10 shows a second embodiment of a processing unit of the detection circuit shown in FIG. 4;

(10) FIGS. 11, 13 and 16 show details of the processing unit of FIG. 10 according to embodiments of the present disclosure;

(11) FIGS. 12a and 12b show waveforms which may occur in the processing unit of FIG. 10 in embodiments of the present disclosure;

(12) FIGS. 14 and 15 show further details of embodiments of the active discharge circuit of FIG. 3; and

(13) FIG. 17 is a flowchart illustrating the operation of the discharge circuit of FIGS. 2 and 3 according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

(14) In the following description, numerous specific details are given to provide a thorough understanding of embodiments of the present disclosure. The embodiments can be practiced without one or several of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the described embodiments.

(15) Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

(16) The headings provided herein are for convenience only and should not limit the scope or meaning of the embodiments.

(17) In the following figures parts, elements or components which have already been described with reference to FIG. 1 are denoted by the same references previously used in such figure. The description of such previously described elements will not be repeated in the following in order not to overburden the present detailed description.

(18) As mentioned in the foregoing, the present disclosure provides solutions for discharging an X-capacitor.

(19) FIG. 2 shows an embodiment of a device 30 in accordance with the present disclosure.

(20) Similar to the devices shown in FIGS. 1a and 1b, the device 30 includes a connector 302, such as a plug, comprising at least two power supply terminals 302a and 302b for connection to the external AC power supply 20 and the AC power supply signal Vin received via the terminals 302a and 302b is provided to a load.

(21) For example, in an embodiment, the AC power supply signal received via the terminals 302a and 302b is provided to a rectifier 310, such as a bridge rectifier, which converts the AC power supply signal to a DC power signal, which is provided via a positive power line 316a and a negative power line 316b, which represents a ground GND, to a DC load 312.

(22) For example, in an embodiment, the DC load may be a DC/DC or DC/AC switching converter 312, which provides a regulated power supply signal to an external and/or internal load indicated with the reference signs 314a and 314b, respectively. For example, typical topologies for switching converters are buck, boost, buck-boost, flyback, forward, half-bridge or full-bridge converters, which are well known to those skilled in the art, rendering a detailed description herein unnecessary.

(23) In the embodiment considered, the device 30 also includes an X-capacitor 304, i.e., at least one capacitor being connected (e.g., directly) between the terminals 302a and 302b.

(24) In the embodiment considered, the device 30 comprises further an active discharge circuit 308 configured to selectively discharge the capacitor 304.

(25) FIG. 3 shows an embodiment of an active discharge circuit 308 in accordance with one embodiment of the present disclosure.

(26) Specifically, in the embodiment considered, the active discharge circuit 308 comprises a detection circuit 40 configured to determine whether the connector 302 has been disconnected from the AC power source 20 and a discharge circuit 50 driven via a signal EN provided by the detection circuit 40 and configured to discharge the capacitor 304 when the signal EN indicates that the connector 302 has been disconnected from the AC power source 20.

(27) Generally, when the device 30 is connected to the AC voltage source 20, the voltage V.sub.X at the capacitor 304, i.e., the voltage between the terminals 302a and 302b, is a sinusoidal signal, i.e., an oscillation having a given amplitude and frequency, e.g., an amplitude of 230V and a frequency of 50 Hz.

(28) Thus, by detecting an AC oscillation at the capacitor 304, the detection circuit 40 may determine whether the device 30 is connected to the AC power source 20 or not.

(29) FIG. 4 shows a possible embodiment of the detection circuit 40.

(30) In the embodiment considered, the detection circuit 40 comprises an optional rectifier circuit 402, a voltage sensor 404 and a processing unit 406.

(31) Specifically, the optional rectifier circuit 402 is interposed between the capacitor 304 and the voltage sensor 404, wherein the rectifier circuit 402 is configured to convert the AC voltage signal V.sub.X at the capacitor 304 into a DC voltage signal.

(32) Conversely, the voltage sensor 404 is configured to measure the voltage at the output of the rectifier circuit 402 (or in alternative directly at the capacitor 304). Accordingly, generally, a signal S provided at the output of the voltage sensor 404 is representative of the voltage V.sub.X at the capacitor 304.

(33) For example, FIG. 14 shows a possible embodiment of the voltage sensor 404, which is particularly suitable for devices comprising already a rectifier 310, such as a bridge rectifier.

(34) In the embodiment considered, the voltage V.sub.X at the capacitor 304 is rectified via a rectifier circuit 402. Specifically, in the embodiment considered, the rectifier comprises two diodes D.sub.1 and D.sub.2. More specifically, the anode of the diode D1 is connected to a first terminal of the capacitor 304, e.g., to the terminal 302a, and the anode of the diode D2 is connected to the second terminal of the capacitor 304, e.g., to the terminal 302b. The cathodes of the diodes D.sub.1 and D.sub.2 are connected (preferably directly) together and provide thus always a positive voltage. In particular, merely two diodes D.sub.1 and D.sub.2 are sufficient, because the ground GND provided by the rectifier 310 may be used as negative reference for this voltage.

(35) Conversely, e.g., in case the rectifier 310 is missing, a full bridge rectifier could be used in the rectifier circuit 402.

(36) The positive voltage provided at the cathodes of the D.sub.1 and D.sub.2 is provided to the voltage sensor 404.

(37) For example, in the embodiment considered, a voltage divider comprising two resistors R1 and R2 connected in series is used as voltage sensor 404. Specifically, in the embodiment considered the voltage divider is connected between the connection point of the cathodes of the diodes D.sub.1 and D.sub.2 and the ground GND, and the intermediate point between the resistors R1 and R2 provides the sensor signal S. Accordingly, in the embodiment considered, the signal S corresponds to a positive voltage with respect to the ground GND and corresponds to a rectified and scaled down version of the voltage V.sub.X at the capacitor 304.

(38) Thus, when the device 30 is connected to the AC power source 20 the signal S may have different waveforms which primarily depend on the presence and implementation of the rectifier circuit 402. For example, generally, the signal S may be an AC sinusoidal oscillation, a positive sinusoidal oscillation (e.g., by adding a DC offset to the AC oscillation), a rectified sinusoidal oscillation, or a waveform comprising only each second half-wave (e.g., by using only a single diode, e.g., diode D.sub.1 or D.sub.2, in the rectifier circuit 402).

(39) Finally, the processing unit 406, which may be implemented by any suitable analog and/or digital circuit, elaborates the sensor signal S and determines the signal EN as a function of the sensor signal S.

(40) Generally, the discharge circuit 50 (FIG. 3) includes at least one electronic switch SW configured to selectively discharge the capacitor 304 as a function of the signal EN. In order to limit the discharge current, a resistor, or generally a resistive element R, or a current generator I (generating a linear/non-linear current and/or a constant/non-constant current) could also be connected in series with such an electronic switch SW, as represented through the dashed line depictions of these elements in FIG. 15. The discharge circuit 50 may include the electronic switch SW connected to the resistive element R and/or the current generator I. Accordingly, such an electronic switch SW could be connected, e.g., in parallel with the capacitor 304.

(41) Conversely, if the device 30 includes a rectifier 310, the electronic switch SW could also discharge the capacitor 304 to ground GND.

(42) For example, FIG. 15 shows an embodiment of the discharge circuit 50 which may be used in combination with the voltage sensor 404 shown in FIG. 14. In fact, in this embodiment, the connection point of the cathodes of the diodes D.sub.1 and D.sub.2 always provides a positive voltage with respect to the ground GND, i.e., the negative line at the output of the rectifier 310. Accordingly, in this case an electronic switch SW, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), e.g., with n channel, may be connected between the connection point of the cathodes of the diodes D.sub.1/D.sub.2 and ground GND, wherein the control gate of the electronic switch SW is driven via the signal EN.

(43) Generally, also in this case a disconnection event should not be detected by simply checking whether the signal S is greater than a given threshold, because when the connector 302 (FIG. 3) is disconnected, the capacitor 304 may be charged to any value between zero and the maximum line voltage. Therefore the use of a constant fixed threshold is not a reliable way to implement the detection.

(44) Moreover, detection is complex also because possible further components of an EMI filter placed upstream the connector 302 could introduce distortions in the sensed voltage S. Furthermore, often it is also not possible to exactly predict the distortion, which often depends on the operative conditions of the electric load of the device.

(45) Finally, in particular in the context of switched mode power supplies, the sensor signal S could also not decrease gradually when the device 30 is disconnected, but the subsequent electronic converter 310 could still consume energy leading to voltage profiles similar to a linear or step-down slow discharge.

(46) For example, FIGS. 5a and 5b show two possible waveforms of the signal S, in the case of a switched mode power supply with or without output load, respectively.

(47) Specifically, in the examples considered, once the plug is disconnected at a time to, the discharge time of the capacitor 304 varies significantly between the waveforms shown in FIGS. 5a and 5b. For example, the discharge behavior shown in FIG. 5a is quite similar to a decreasing sinusoidal waveform.

(48) Accordingly, a disconnection event could be detected by comparing the maximum peak of the AC voltage, which corresponds to the amplitude of the oscillation, with a given voltage reference. However, this solution would require the knowledge of the nominal amplitude of the AC voltage, which may also vary from 80 VAC to 260 VAC for different countries.

(49) Thus, generally, the voltage at the capacitor 304 could be compared with a fixed lower voltage reference, such as 70 V. However, additional components, in particular inductors in the EMI filter, may introduce distortions in the signal S.

(50) For example, this may be particularly relevant, when the signal S is measured with respect to the ground GND downstream the rectifier 310 as shown, e.g., in FIG. 15. In this case, the shape of the sensed signal S is also strongly dependent on the load conditions.

(51) In this context, the inventors have observed that the peak value is almost defined by the AC mains peak value, but the lowest value (the so called valley value) depends strongly on the load conditions. For example, often this is due to the finite (and different from zero) value of the EMI filter impedance. Accordingly, in the case of a heavy load the valley value of signal S would be low, even close to zero. On the other hand, the valley value will be higher at light load conditions. Because of this a fixed low threshold could lead to a missed or wrong detection of the AC mains (with an undesired activation of the X-capacitor discharge).

(52) FIG. 6 shows a first embodiment of a processing unit 406a in accordance with the present disclosure.

(53) Specifically, in the embodiment considered, instead of using a fixed and preconfigured voltage reference, the solution determines a dynamic voltage threshold.

(54) Specifically, in the embodiment considered, the processing unit 406a comprises comparator circuit 412a, a dynamic threshold generator circuit 416a, an elaboration circuit 418a and a timer circuit 414a.

(55) Specifically, the dynamic threshold generator circuit 416a is configured to provide one or more threshold values for the comparator circuit 412a as a function of the signal S.

(56) For example, in the embodiment considered, the dynamic threshold generator circuit 416a comprises a peak detector 408 and a threshold generator circuit 410.

(57) Specifically, the peak detector 408 is configured to detect the maximum value in the signal S. For example, suitable peak detectors are described in the patent application U.S. Ser. No. 14/510,925 filed on Oct. 9, 2014, which is incorporated for this purpose herein by reference in its entirety.

(58) Accordingly, when the device 30 is connected to the AC power source 20, the peak detector 408 will provide after one or more oscillations of the AC power signal the maximum value of the signal S, which represents the amplitude of the oscillation of the voltage V.sub.X at the capacitor 304. Accordingly, this peak value represents an upper dynamic threshold DHT for the signal S.

(59) In the embodiment considered, the threshold generator circuit 410 uses this signal in order to generate at least one threshold value, such as a reference voltage signal, for the comparator circuit 412a.

(60) For example, in an embodiment, the comparator circuit 412a comprises a single comparator configured to compare the signal S with a lower threshold DLT. In this case, the threshold generator circuit 410 may be configured to determine dynamically this lower threshold value DLT as a function of the upper dynamic threshold DHT. For example, the lower threshold DLT may be calculated by subtracting a given value from the threshold DHT, such as 20-50V, or by scaling the threshold DHT with a given percentage, such as 90%. Generally, the difference between the two thresholds may also be programmable, e.g., by means of software, trimming or metal options.

(61) Accordingly, the lower threshold value DLT corresponds to a dynamic threshold which is determined as a function of the peak value of the voltage at the capacitor 304. Accordingly, in the embodiment considered, a signal OVTH at the output of the comparator 412a indicates weather the signal S, being indicative of the voltage at the capacitor 304, is greater than the lower threshold DLT.

(62) For example, FIG. 7 shows a possible waveform for the signal OVTH which is set to a first logic level, e.g., “1”, when the voltage at the capacitor 304 is greater than the lower threshold DLT and to a second logic level, e.g., “0”, when the voltage at the capacitor 304 is smaller than the lower threshold DLT. Accordingly, as long as the AC power source 20 is connected to the device 30, a single pulse of the OVTH signal is generated for each half-wave of the rectified oscillation of the AC power source 20. Conversely, in the absence of a rectifier 310 or when using only a single diode in the rectifier circuit 310, a single pulse would be generated for each oscillation.

(63) In an embodiment, the comparator circuit 412a may comprise instead a window comparator configured to determine whether the signal S is between a lower and an upper threshold. In this case, the threshold generator circuit 410 may be configured to provide both the lower dynamic threshold DLT and the upper dynamic threshold DHT to the window comparator of the comparator circuit 412a. Accordingly, in this case, the signal OVTH would indicate whether the signal S is between the lower and the upper dynamic thresholds DLT and DHT. For example, such a window comparator may improve robustness and effectiveness, and may be suitable when AC variations should be detected with higher precision/resolution.

(64) Thus, when the device 30 is connected to the AC power source 20, the signal OVTH at the output of the comparator circuit 412a will comprise at least one pulse for a time period T corresponding to:
T=1/f.sub.AC.
where f.sub.AC is the frequency of the AC oscillation of the AC power source 20, which is usually 50 or 60 Hz.

(65) Conversely, such pulses will be missing, when the device 30 is disconnected from the AC power source 20.

(66) For example, FIG. 8 shows in this respect possible waveforms for the signal S and a corresponding signal OVTH.

(67) Accordingly, in the embodiment considered, the timer circuit 414a is used to determine, similar to a watchdog timer or timeout counter, whether a given time period has lapsed since the last pulse occurred in the signal OVTH.

(68) For example, such a timer circuit 414a may be implemented with a counter, which increases or decreases a count value until a given value has been reached and which is reset to a given initial value based on a reset signal RESET. Accordingly, in this case, such a timer circuit 414a may be reset as a function of the signal OVTH.

(69) For example, in the embodiment considered, the signal OVTH is provided for this purpose to the elaboration circuit 418a, which determines a signal RESET for the timer circuit 414a as a function of the signal OVTH.

(70) For example, in an embodiment, the elaboration circuit 418a is configured (based on the logic values of the signal OVTH) to reset or restart the timer circuit 414a at each raising edge of the signal OVTH, which represents a positive slope in the signal S. Accordingly, in this case, the timer circuit 414a may determine whether a given time period has lapsed since a last raising edge of the signal OVTH. For example, typically the time threshold or timeout value TO for the timer circuit 414a should correspond to several periods T of a typical oscillation of the AC power source, such as 40-100 ms. Accordingly, when the timer circuit 414a reaches the timeout value TO, the timer circuit 414a may enable the discharge circuit 50 via the signal EN in order to discharge the capacitor 304.

(71) In an embodiment, the elaboration circuit 418a may be configured to reset the timer circuit 414a also at each falling edge of the signal OVTH. Accordingly, in this case, the timer circuit 414a is configured to enable the discharge circuit when the timer circuit reaches a given timeout value TO since the last raising or falling edged of the signal OVTH.

(72) Thus generally, the timer circuit 414a enables the discharge circuit when a given time period has lapsed since the last raising and/or falling edge in the signal OVTH.

(73) Conversely, different solutions may be used to determine when the discharge circuit 50 should be disabled or deactivated again.

(74) For example, the timer circuit 414a could be configured to deactivate the discharge circuit 50 only at a raising edge in the signal OVTH, which indicates a positive slope in the signal S. For example, in this case, the elaboration circuit 418a may be configured to reset the timer circuit 414a via the signal RESET at each raising edge in the signal OVTH.

(75) Conversely, the timer circuit 414a should not simply disable the discharge circuit 50 at a falling edge, because once the discharge circuit is enabled, the voltage at the capacitor 304 decreases, which could cause a falling edge in the signal OVTH.

(76) Thus, in an embodiment, in order to avoid this problem, the elaboration circuit 418a is configured to determine whether the signal S shows slope changes (second order derivative) and eventually resets the timer circuit 414a.

(77) For example, in an embodiment, the elaboration circuit 418a is configured to determine the time elapsed between two consecutive edges in the signal OVTH, i.e., between a raising edge and a falling edge or vice versa between a falling edge and a raising edge, thereby determining two values: a first value TH indicating the duration in which the signal OVTH was high and second value TL indicating the duration in which the signal OVTH was low. In this case, the elaboration circuit 418a may be configured to compare these durations with at least one time threshold value in order to determine whether these durations are within given limits. For example, in an embodiment the elaboration circuit 418a may determine whether the first duration TH and the second duration TL are both smaller than a timeout value TO, which could correspond to the duration of several periods or sub-periods of a typical AC oscillation, e.g., 5, 10, 20-100 ms. Accordingly, in this case, the elaboration circuit 418a could reset the timer circuit 414a thus disabling the discharge circuit 50 only when the durations TH and/or TL are within the specified limits.

(78) For example, FIG. 9 shows for this embodiment a possible waveform for the signal OVTH and a respective enable signal EN for the discharge circuit 50.

(79) Specifically, in the example considered: at a time t.sub.1 the voltage of the signal S reaches the lower voltage threshold DLT, at a time t.sub.3 the voltage of the signal S falls below the lower voltage threshold DLT, at a time t.sub.4 the voltage of the signal S again reaches the lower voltage threshold DLT, and at a time t.sub.5 the voltage of the signal S again falls below the lower voltage threshold DLT.

(80) Accordingly, in the embodiment considered, the signal OVTH includes two pulses. Moreover, in the example considered, the duration TH.sub.1 of the first pulse and the duration TL.sub.1 are greater than the timeout value TO and the duration TH.sub.2 of the second pulse is smaller than the timeout value TO. Accordingly, considering the above configuration of the elaboration circuit 418a, the timer circuit 414a will enable the discharge circuit 50 once the time TO has elapsed since the first raising edge, i.e., at a time t.sub.2. Conversely, the elaboration circuit 418a will reset the timer circuit 414a, thereby deactivating the discharge circuit 50, only at the second falling edge at time t.sub.5, because only at this moment the elaboration circuit 414 is able to determine the duration TH.sub.2 of the second pulse and compare this duration with the timeout value TO.

(81) This method has the further advantage that low variation of AC input voltage may be filtered automatically.

(82) FIG. 10 shows a second embodiment of the processing unit 406b.

(83) Specifically, also in this embodiment, the processing unit 406b comprises comparator circuit 412b, a dynamic threshold generator circuit 416b, an elaboration circuit 418b and a timer circuit 414b. Also in this case, the dynamic threshold generator circuit 416b is configured to provide at least one threshold value to the comparator circuit 412b, which has been determined as a function of the signal S, and the elaboration circuit 418b is configured to reset the timer circuit 414b as a function of one or more signals at the output of the comparator circuit 412b.

(84) However, while in the embodiment shown in FIG. 6, the threshold value(s) were determined in a feed-forward or open-loop manner directly based on the signal S, in the embodiment considered, the threshold values are updated in a feed-back or closed loop manner based on the signals at the output of the comparator circuit 412b.

(85) Specifically, in the embodiment considered, the comparator circuit 412b comprises two comparators 412.sub.1 and 412.sub.2, which compare the signal S with two dynamic threshold values DT.sub.1 and DT.sub.2 being provided by the dynamic threshold generator circuit 416b.

(86) Accordingly, in the embodiment considered, the signal COMP.sub.1 at the output of the first comparator 412.sub.1 indicates weather the signal S is greater than the voltage reference signal DT.sub.1 and the signal COMP.sub.2 at the output of the second comparator 412.sub.2 indicates weather the signal S is greater than the voltage reference signal DT.sub.2.

(87) In the embodiment considered, the comparator circuit 412b provides the comparison signals COMP.sub.1 and COMP.sub.2 to the elaboration circuit 418b, such as a combinational circuit, which is configured to determine, based on the signals COMP.sub.1 and COMP.sub.2 whether the signal S is increasing or decreasing.

(88) Specifically, in the embodiment considered, the elaboration circuit 418b provides for this purpose a signal INC indicating that the signal S is greater than both DT.sub.1 and DT.sub.2, and a signal DEC indicating that the signal S is smaller than both DT.sub.1 and DT.sub.2.

(89) For example FIG. 11 shows a possible embodiment of the elaboration circuit 418b for the logic levels of the signals COMP.sub.1 and COMP.sub.2 which result from the connection of the signals to the positive and negative input terminals of the comparators 412.sub.1 and 412.sub.2 as shown in FIG. 10. Specifically, in this case, the signal INC may be obtained via an AND gate 420 and the signal DEC may be obtained via an NOR gate 422, wherein both gates receive the signals COMP.sub.1 and COMP.sub.2 as input.

(90) Accordingly, the dynamic threshold generator circuit 416b is able to determine (via the comparator circuit 412b) whether the signal S is greater than (e.g., INC=“1” and DEC=“0”), smaller than (e.g., INC=“0” and DEC=“1”) or between the threshold values DT.sub.1 and DT.sub.2 (e.g., INC=“0” and DEC=“0”). In the embodiment considered, the dynamic threshold generator circuit 416b uses this information in order to vary the thresholds DT.sub.1 and DT.sub.2.

(91) For example, FIG. 16 shows a possible embodiment of dynamic threshold generator circuit 416b. In the embodiment considered, the signals INC and DEC are provided to a digital processing unit 420, such as a microprocessor. The digital processing unit 420 elaborates the signals INC and DEC and sets the threshold values DT.sub.1 and DT.sub.2 via respective digital-to-analog converters DAC.sub.1 and DAC.sub.2.

(92) FIG. 12a shows in that respect a possible operation of an embodiment of the dynamic threshold generator circuit 416b.

(93) In the embodiment considered, initially the threshold DT.sub.1 and DT.sub.2 are set to respective initial values, e.g., DT.sub.1=DT.sub.1,0 and DT.sub.2=DT.sub.2,0, wherein one of the thresholds is greater than the other one, i.e., one threshold represents a lower threshold and the other threshold represents a higher threshold, e.g., DT.sub.2,0>DT.sub.1,0.

(94) Once the signal INC indicates that the signal S is greater than DT.sub.1 and DT.sub.2, i.e., S>DT.sub.1,0 and S>DT.sub.2,0, the circuit 416b assigns a new value to the lower threshold, wherein the new value is greater than the previous higher threshold. For example, in the example considered, the threshold DT.sub.1 is assigned a new value DT.sub.1,1, with DT.sub.1,1>DT.sub.2,0. Thus the previous lower threshold becomes the new higher threshold (being greater than the current value of the signal S) and intrinsically the signal INC changes again the logic state.

(95) This scheme is repeated each time the signal INC goes to high, thereby following the raising slope of the signal S. For example, in the embodiment considered, the signal S exceeds three times the thresholds DT.sub.1 and DT.sub.2 till the thresholds are set to DT.sub.1=DT.sub.1,2 and DT.sub.2=DT.sub.2,1.

(96) Accordingly, in the embodiment considered, one of the two thresholds represents a lower threshold and the other represents a higher threshold and when the signal INC indicates that the signal S is greater than both thresholds, the circuit 416b increases the lower threshold value such that the lower threshold value becomes the new higher threshold value.

(97) Conversely, when the signal S is decreasing again, at a given moment the signal DEC will show that the signal S is smaller than DT.sub.1 and DT.sub.2, i.e., S<DT.sub.1,2 and S<DT.sub.2,1. At this moment, the circuit 416b will decrease the current higher threshold. For example, in FIG. 12a, the signal DT.sub.1 will be set to a new value DT.sub.1,1, which is smaller than the current value of DT.sub.2.

(98) Accordingly, in the embodiment considered, when the signal DEC indicates that the signal S is smaller than both thresholds, the circuit 416b decreases the higher threshold value such that the higher threshold value becomes the new lower threshold value.

(99) Thus, by repeating the above operation, the processing unit 406b is able to follow the waveform of the signal S. Moreover, the signals INC and DEC show respectively whether the signal S has a positive slop or a negative slope.

(100) In an embodiment, at least four levels are used for each of the thresholds DT.sub.1 and DT.sub.2. For example, FIG. 13 shows a table of typical values of 7 levels L for each of the thresholds DT.sub.1 and DT.sub.2. Specifically, these values are suitable both for 110 VAC and 230 VDC applications. Those of skill in the art will appreciate that the values indicated relate to the absolute threshold values with respect to the amplitude of the voltage V.sub.X, which eventually should be scaled based on the ratio of the voltage detection circuit 404b.

(101) Generally, as shown in FIG. 12a, when the signal S decreases to zero, the thresholds may be maintained at the lowest level, e.g., levels DT.sub.1,0 and DT.sub.2,0, which are greater than zero. Accordingly, in this case, the signal DEC will remain high until the signal S increases again exceeding the lower threshold, e.g., DT.sub.1,0 in FIG. 12a.

(102) Conversely, FIG. 12b shows an embodiment, in which the processing unit 406b assigns in this case a default value DT.sub.0, being at most zero or even negative. Specifically, when the threshold values have reached the lowest levels, e.g., levels DT.sub.1,0 and DT.sub.2,0, and the signal DEC goes to high, the processing unit assigns to the current higher threshold this default value, e.g., DT.sub.2=DT.sub.0.

(103) Thus, generally, the levels of the thresholds signals DT.sub.1 and DT.sub.2 are configured such that the signals INC and DEC comprise (in the presence of an AC power supply signal) a plurality of pulses for each raising or falling slope of the signal S respectively.

(104) Accordingly, similar to the previous embodiment shown in FIG. 6, the elaboration circuit 418b may be configured to reset the timer circuit 414b via a signal RESET as a function of the signals provided by the comparator circuit 412b, i.e., the signals INC and DEC.

(105) For example, in the embodiment shown in FIG. 11, the signal RESET corresponds to the signal INC, i.e., the signal RESET comprises only the pulses of the signal INC, showing in this way a positive slope of the signal S.

(106) However, while in the previous embodiment was required some kind of logic in order to analyze the waveform of the signal OVTH, e.g., in order to determine the raising edge of the signal OVTH, in this case, the signal INC comprises only short pulses and accordingly this signal could be used directly to reset to timer circuit 414b.

(107) Generally, the embodiments shown with respect to FIGS. 6 and 10 may also be combined and components may be reused, e.g., only a single timer circuit may be used instead of two separate timer circuits 414a and 414b. For example, in this case, the signal RESET provided by the elaboration circuit 418a of FIG. 6 could be combined, e.g., via a logic OR gate, with the signal RESET provided by the elaboration circuit 418b of FIG. 10.

(108) FIG. 17 is a flowchart illustrating the operation of the discharge circuit 308 of FIGS. 2 and 3 according to one embodiment of the present disclosure. The method of detecting whether an AC oscillation is applied to an X capacitor of a device begins in step 1700 and proceeds to step 1702 monitors a voltage at said X capacitor and proceeds to step 1704 and generate at least one comparison signal indicating whether said voltage at said X capacitor is contained in a voltage window defined by a lower threshold value and an upper threshold value. From step 1704 the process goes to step 1706 and varies said upper and lower threshold values as a function of said monitored voltage at said X capacitor. The process then goes to step 1708 and detects at least one of leading and falling edges of the comparison signal and then goes to step 1710 and increments a count value over time starting from an initial value for the count value. From step 1710 the process goes to step 1712 and resets the count value to the initial value as a function of the detected at least one of leaving and falling edges of the comparison signal. After step 1712 the process goes to step 1714 and determines whether the count value has reached a time threshold value and then goes t step 1716 and discharges the X capacitor responsive to the time count reaching the time threshold value. The process then terminates in step 1718. In one embodiment, step 1701 includes generating a sensor signal responsive to the voltage at said X capacitor and determining the upper threshold value indicative of a peak value of said sensor signal and determining the lower threshold value as a function of said upper threshold value. The count value in one embodiment indicates a time between leading and falling edges of the comparison signal. In one embodiment, varying said at least one threshold as a function of said monitored voltage at said X capacitor comprises varying the at least one threshold value as a function the comparison signal. In an embodiment, the comparison signal comprises first and second comparison signals and the method further includes generating a first comparison signal indicating whether said monitored voltage at said X capacitor is greater than a first threshold value and generating a second comparison signal indicating whether said monitored voltage at said X capacitor is greater than a second threshold value.

(109) Such a combination of the circuits provides a system solution suitable for a wide range of applicative conditions (e.g., EMI filter structures, load conditions, etc.). The circuit combination is a reliable and effectiveness solution regardless system operating conditions.

(110) Of course, without prejudice to the principles of the present disclosure, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated herein purely by way of example, without thereby departing from the scope of the present disclosure.

(111) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.